a) Field of the Invention
The present invention relates to an ion sensor suitable for analysis of ion species contained in a biological liquid, and more particularly to an ion sensor suitable for analysis of potassium ion, sodium ion, halide ion or carbonate ion by potentiometric determination.
b) Related Art
The ion sensor can selectively determine concentrations of specific ion species in a solution and has been employed in various fields including concentration monitoring of specific ion species, water quality analysis, etc. Particularly in the medical field it is applied to quantitatively determine ion species in blood or biological liquids such as urine, etc., for example, chloride ions, potassium ions, etc. Since concentrations of specific ion species in a biological liquid are closely related to metabolic reactions of living bodies, hypertension symptom, kidney disorder, neurosis trouble, etc. are diagnosed by determining concentrations of specific ion species.
Between an activity a of ion species to be determined and an electrode potential level E given by an ion sensor, a correlation that the logarithm of activity a is proportional to a change in the level E is established as shown by the following equation: EQU E=E.sub.0 +2.303(RT/ZF) log a
and the activity a of ion species to be determined can be simply calculated from measurements of the level E. In the foregoing equation, R is a gas constant, T an absolute temperature, Z an ionic factor, F a Faraday constant and E.sub.0 a standard potential of the system. With the ion sensor, concentrations of ion species existing in blood or a biological liquid can be determined in a broad concentration range only by measuring the potential level.
Generally, an ion sensor comprises an internal solid electrode, an ion selective membrane, an internal solution, where an agar gel containing a supporting electrolyte is used as the inner solution serving to conduct electricity between the ion-selective membrane and the inner electrode. Among ion sensors, an ion sensor in such a structure that the ion selective membrane is directly provided on the internal solid electrode without using any internal solution is called coated wire electrode (CWE). CWE is simple in the electrode preparation, handling, maintenance, etc. and thus has been intensively studied.
Ion sensor disclosed in U.S. Pat. No. 4,214,968 can directly read concentrations of specific ion species as a function of ion activity by spotting a liquid sample without any preparative adjustment, storage by wet process or equilibration.
The above-mentioned CWE generally comprises an ion selective membrane, an internal solid electrode and an electrode body. Since the internal solid electrode is in direct contact with the ion selective membrane, the electrode potential level drifts largely and the electrode potential stability is not satisfactory when used for a prolonged time. Furthermore, the electrode resistance is high and CWE is highly susceptible particularly to changes in temperature.
The art disclosed in said U.S. Pat. No. 4,214,968 provides an ion sensor unnecessitating preparative adjustment, storage by wet process or equilibration, but is not suitable for continuous measurement of liquid samples flowing, for example, along a path and no mention is made of a suitable ion sensor for the continuous measurement at all.